Carbon dioxide (CO2) and carbon monoxide (CO) lasers use transitions between molecular vibrational and rotational states in an excited gas mixture to produce infrared laser-radiation. In a CO2 laser, the gas mixture includes CO2, helium (He), nitrogen (N2), and usually smaller concentrations hydrogen (H2). The gas mixture is energized (pumped) by applying an electric current or a radio-frequency (RF) field between two electrodes. RF pumping has an advantage of longer electrode lifetime. Excited CO2 gas mixtures can emit laser-radiation over a plurality of wavelength ranges (bands), which are centered around 9.3 micrometers (μm), 9.6 μm, 10.2 μm, and 10.6 μm.
In a slab configuration, the gas mixture is energized in a volume between flat wave-guiding surfaces of two closely-spaced electrodes. A laser-resonator is formed around the energized gas mixture by two resonator mirrors, known by practitioners of the art as an “output coupler” or “front mirror” and a “high-reflector” or “rear mirror”. The gas mixture occupies a volume defined in height by the small gap between the electrodes, in length by the distance between the resonator mirrors, and in width by the breadth of the resonator mirrors. In diffusion-cooled configurations, the gas mixture is cooled by heat diffusing to the electrodes, which typically include channels containing a flowing liquid coolant. In fast-flow configurations, cooling is achieved by rapidly circulating the gas mixture in a circuit that includes a gas reservoir, the laser-resonator, and a heat exchanger.
In a slab configuration, the resonator mirrors typically form an unstable laser-resonator. Spontaneously emitted radiation, directed by the resonator mirrors, is amplified by stimulated emission during multiple passes through the energized gas mixture. Output laser-radiation exits the laser-resonator after a final reflection from the high-reflector as an approximately collimated beam, passing through a hole in the output coupler or passing by an outside edge of the output coupler. The hole or edge region through which the beam passes is sealed gas-tight by a transparent window.
CO2 lasers are used primarily for industrial material processing, particularly for cutting, scribing, marking, and welding. Cutting materials such as plastic and wood typically requires tens to hundreds of Watts of power, while cutting and welding metals and metal alloys typically requires kilo-Watts of power, depending on the thickness of the workpiece. The emission band preferred in a specific application depends on the absorption spectrum of the material being processed. For example, the 10.2 μm band is preferred for cutting some types of plastic, while the 9.3 μm band was shown to be preferable for ablation of hard tissue in dental procedures.
Generally, the resonator mirrors in a CO2 laser have a metal surface, which is most commonly copper, or a broadband coating that is reflective at all emission bands between 9 μm and 11 μm. CO2 lasers tend to operate in the dominant 10.6 μm band. It is challenging to generate laser-radiation purely in one emission band, without any spurious emission in one of the other emission bands. It is particularly challenging to generate laser-radiation purely in the 9.3 μm band or 9.6 μm band, which have smaller emission cross-sections.
To generate laser-radiation in just one emission band, at least one resonator mirror may be coated with a band-selective coating, which is highly reflective for the selected emission band and is weakly reflective for the other emission bands. Lasing of the other emission bands is thereby suppressed. Such band-selective coatings are thicker than broadband coatings, having many quarter-wavelength thick layers made of dielectric materials. Mirror designs are optimized for the required spectral selectivity, but such thick coatings are prone to particle-induced optical damage due to the comparatively low thermal conductivity of the dielectric materials. Localized heating caused by absorption of laser-radiation by a particle on the coating surface can induce catastrophic damage. Such thick coatings, having different thermal expansion characteristics from underlying substrate materials, are also prone to delamination. Another disadvantage of band-selective coating is high cost compared to simpler broadband coatings.
It is known that the longer wavelength 10.2 μm and 10.6 μm bands can be suppressed by applying a passivation layer of SiO2 to the flat wave-guiding surface of at least one of the electrodes and precisely setting the distance between the electrodes. Such an arrangement is described in U.S. Pat. No. 8,331,416 and can be made to generate laser-radiation in the 9.3 μm band, but does not provide stable operation in just the 9.6 μm band or 10.2 μm band.
There is need for a high-power CO2 laser reliably producing laser-radiation in just one selected emission band, which is cost-effective to manufacture and not prone to optical damage. Preferably, such a CO2 laser would be capable of producing laser-radiation purely in any one of the emission bands between 9 μm and 11 μm, with the output emission band selectable during manufacture or operation thereof.